Made session restore block about:flags driven browser restart.

This fixes a regression that's triggered by user password change where subsequent GAIA sign in flow wouldn't preserve OAuth2 refresh token anymore - see the BUG for more details.

BUG=289965
TEST=manual
R=xiyuan@chromium.org

Review URL: https://codereview.chromium.org/24473002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225294 0039d316-1c4b-4281-b951-d872f2087c98